Buggy code:
public class SparseRealVector implements RealVector { private static final long serialVersionUID = 8772222695580707260L ; public static final double DEFAULT_ZERO_TOLERANCE = 1.0E-12 ; private final OpenIntToDoubleHashMap entries ; private final int virtualSize ; private double epsilon ; public SparseRealVector ( ) { } public SparseRealVector ( int dimension ) { } public SparseRealVector ( int dimension , double epsilon ) { } protected SparseRealVector ( SparseRealVector v , int resize ) { } public SparseRealVector ( int dimension , int expectedSize ) { } public SparseRealVector ( int dimension , int expectedSize , double epsilon ) { } public SparseRealVector ( double [ ] values ) { } public SparseRealVector ( double [ ] values , double epsilon ) { } public SparseRealVector ( Double [ ] values ) { } public SparseRealVector ( Double [ ] values , double epsilon ) { } public SparseRealVector ( SparseRealVector v ) { } public SparseRealVector ( RealVector v ) { } private OpenIntToDoubleHashMap getEntries ( ) { } protected boolean isZero ( double value ) { } public double getEpsilon ( ) { } public void setEpsilon ( double epsilon ) { } public RealVector add ( RealVector v ) throws IllegalArgumentException { } public SparseRealVector add ( SparseRealVector v ) throws IllegalArgumentException { } public RealVector add ( double [ ] v ) throws IllegalArgumentException { } public SparseRealVector append ( SparseRealVector v ) { } public RealVector append ( RealVector v ) { } public RealVector append ( double d ) { } public RealVector append ( double [ ] a ) { } public RealVector copy ( ) { } public double dotProduct ( RealVector v ) throws IllegalArgumentException { } public double dotProduct ( double [ ] v ) throws IllegalArgumentException { } public RealVector ebeDivide ( RealVector v ) throws IllegalArgumentException { } public RealVector ebeDivide ( double [ ] v ) throws IllegalArgumentException { checkVectorDimensions ( v . length ) ; SparseRealVector res = new SparseRealVector ( this ) ; Iterator iter = res . entries . iterator ( ) ; while ( iter . hasNext ( ) ) { iter . advance ( ) ; res . setEntry ( iter . key ( ) , ( ( iter . value ( ) ) // ( v [ iter . key ( ) ] ) ) ) ; } <START_BUG> return null ; <END_BUG> } public RealVector ebeMultiply ( RealVector v ) throws IllegalArgumentException { } public RealVector ebeMultiply ( double [ ] v ) throws IllegalArgumentException { } public RealVector getSubVector ( int index , int n ) throws MatrixIndexException { } public double [ ] getData ( ) { } public int getDimension ( ) { } public double getDistance ( SparseRealVector v ) throws IllegalArgumentException { } public double getDistance ( RealVector v ) throws IllegalArgumentException { } public double getDistance ( double [ ] v ) throws IllegalArgumentException { } public double getEntry ( int index ) throws MatrixIndexException { } public double getL1Distance ( SparseRealVector v ) { } public double getL1Distance ( RealVector v ) throws IllegalArgumentException { } public double getL1Distance ( double [ ] v ) throws IllegalArgumentException { } public double getL1Norm ( ) { } private double getLInfDistance ( SparseRealVector v ) { } public double getLInfDistance ( RealVector v ) throws IllegalArgumentException { } public double getLInfDistance ( double [ ] v ) throws IllegalArgumentException { } public double getLInfNorm ( ) { } public double getNorm ( ) { } public boolean isInfinite ( ) { } public boolean isNaN ( ) { } public RealVector mapAbs ( ) { } public RealVector mapAbsToSelf ( ) { } public RealVector mapAcos ( ) { } public RealVector mapAcosToSelf ( ) { } public RealVector mapAdd ( double d ) { } public RealVector mapAddToSelf ( double d ) { } public RealVector mapAsin ( ) { } public RealVector mapAsinToSelf ( ) { } public RealVector mapAtan ( ) { } public RealVector mapAtanToSelf ( ) { } public RealVector mapCbrt ( ) { } public RealVector mapCbrtToSelf ( ) { } public RealVector mapCeil ( ) { } public RealVector mapCeilToSelf ( ) { } public RealVector mapCos ( ) { } public RealVector mapCosToSelf ( ) { } public RealVector mapCosh ( ) { } public RealVector mapCoshToSelf ( ) { } public RealVector mapDivide ( double d ) { } public RealVector mapDivideToSelf ( double d ) { } public RealVector mapExp ( ) { } public RealVector mapExpToSelf ( ) { } public RealVector mapExpm1 ( ) { } public RealVector mapExpm1ToSelf ( ) { } public RealVector mapFloor ( ) { } public RealVector mapFloorToSelf ( ) { } public RealVector mapInv ( ) { } public RealVector mapInvToSelf ( ) { } public RealVector mapLog ( ) { } public RealVector mapLog10 ( ) { } public RealVector mapLog10ToSelf ( ) { } public RealVector mapLog1p ( ) { } public RealVector mapLog1pToSelf ( ) { } public RealVector mapLogToSelf ( ) { } public RealVector mapMultiply ( double d ) { } public RealVector mapMultiplyToSelf ( double d ) { } public RealVector mapPow ( double d ) { } public RealVector mapPowToSelf ( double d ) { } public RealVector mapRint ( ) { } public RealVector mapRintToSelf ( ) { } public RealVector mapSignum ( ) { } public RealVector mapSignumToSelf ( ) { } public RealVector mapSin ( ) { } public RealVector mapSinToSelf ( ) { } public RealVector mapSinh ( ) { } public RealVector mapSinhToSelf ( ) { } public RealVector mapSqrt ( ) { } public RealVector mapSqrtToSelf ( ) { } public RealVector mapSubtract ( double d ) { } public RealVector mapSubtractToSelf ( double d ) { } public RealVector mapTan ( ) { } public RealVector mapTanToSelf ( ) { } public RealVector mapTanh ( ) { } public RealVector mapTanhToSelf ( ) { }

Fix:
return res ;